Gravity measurements are used in several industrial applications. For example, by precisely measuring the local gravitational acceleration at a predetermined point on the earth's surface, an inference can be made regarding the composition of the material beneath the point, and in particular whether the material beneath the point contains oil. In other words, precisely measuring the gravitational acceleration, frequently denoted by the symbol "g", can aid in oil exploration.
Additionally, precise measurements of local gravitational acceleration "g" can be used in many scientific applications, and in particular the study of plate tectonics. Plate tectonics is a branch of geoscience which hypothesizes that the earth's crust is divided into several large tectonic plates, each of which essentially independently floats on the earth's mantle. Under the plate tectonics hypothesis, seismic and volcanic activity result from the movements of tectonic plates relative to each other. Because, among its other attributes, plate tectonics appears to be a viable explanation of the causes of devastating earthquakes and volcanic activity, much scientific research has been undertaken to better understand plate tectonics.
It happens that a knowledge of the magnitude of the gravitational acceleration at various locations on the earth helps to gain insight into plate tectonics. For example, details of magma injection and fault rupture can be investigated by mapping vertical displacements of benchmarks on the surface of the earth. Of course, the magnitude of the gravitational acceleration associated with a particular benchmark changes as the distance between the benchmark and the center of the earth changes. Accordingly, gravity meters for precisely determining the magnitude of the gravitational acceleration at various locations have been introduced for measuring the absolute vertical movement of benchmarks on the surface of the earth.
Preferably, such apparatus are portable, to facilitate field measurements of the gravitational acceleration. One portable gravity meter is disclosed by Zumberge et al. in the periodical "Metrologia", volume 18, pp. 145-152 (1982). The Zumberge et al. gravity meter disclosed in "Metrologia" includes a Michelson interferometer. A Zeeman laser is used as the light source, and the light beam generated by the laser is split into two so-called "arms", one of which is terminated by a cube corner retroreflector which is allowed to be freely accelerated by the Earth's gravity. The other "arm" is reflected off a non-moving surface, and the two reflected "arms" interfere with each other.
As is well-known in the art, the interference of the two "arms" results in the generation of interference fringes. The characteristics of these interference fringes are representative of the acceleration of the cube corner retroreflector (and, hence, of the gravitational acceleration). More particularly, by measuring the time between fringes, the gravitational acceleration can be determined.
While effective for its intended purpose, it happens that the 1982 Zumberge et al. gravity meter is susceptible to output signal noise attributable to acoustic vibrations (e.g., vibrations caused by seismic activity). More specifically, in the 1982 Zumberge et al. gravity meter, as in most if not all gravity meters, high frequency vibrations affect one "arm" of the interferometer differently that they affect the other "arm", because one "arm" is isolated from such vibrations while the other "arm" (i.e., the "arm" terminated by the retroreflector) is not. This results in output signal noise which reduces the precision of the measurement of the gravitational acceleration. Not surprisingly, to best understand plate tectonics, extremely precise measurements of the gravitational acceleration are desirable.
Further, the above-mentioned Zumberge et al. device, like previous gravity meters, is designed for use on dry land. It happens, however, that many geoscience experiments, as well as a significant mount of oil exploration, require measuring the gravitational acceleration at various points on the sea floor.
Up to now, such measurements have been conducted using pressure sensors and sound sensors. Unfortunately, pressure sensor-based instruments are subject to long-term drift, and instruments using sound sensors present a host of additional difficulties which have never yet been satisfactorily overcome. Hence, it would be desirable to provide a gravity meter for measuring vertical displacement of the sea floor.
It will be appreciated that gravity meters which are to be used in such experiments must be enclosed in watertight pressure vessels, which can be expensive to make, particularly in large sizes. Accordingly, gravity meters used in ocean floor experiments are preferably even more compact than the above-mentioned Zumberge, et al. device, to permit the use of relatively small pressure vessels.
It accordingly is an object of the present invention to provide a gravity meter which reduces output signal noise. Another object of the present invention is to provide a portable gravity meter. Still another object of the present invention is to provide a gravity which is easy to use and cost-effective to manufacture.